The present invention relates to fabricating single-crystal silicon devices, particularly to the fabrication of devices on an insulator substrate, and more particularly to a method for producing transistors in a silicon-on-glass substrate, wherein device components are formed on a silicon substrate, transferred to a glass substrate, and whereafter additional transistors may be formed.
Silicon-on-insulator (SOI) technologies have advanced dramatically in recent years towards the goal of producing thin single-crystal silicon films on insulated substrates. Components such as metal-oxide-semiconductor (MOS) transistors, fabricated in SOI films have the potential for increased mobility, reduced parasitic capacitance and leakage current as well as improved radiation hardness due to reduced junction sidewall area and elimination of bottom junction area. To date, there has been no success in achieving single crystal silicon device fabrication on less expensive glass substrates capable of withstanding temperatures of no more than 600.degree. C. Others have achieved this with expensive glasses, such as Corning 1729 using 800.degree. C. (see L. J. Spangler et al., "A Technology for High-Performance Single-Crystal Silicon-on-Insulator Transistors", IEE Electron Device Letters, Vol. 13, No. 4, April 1987, pp. 137-139) and Corning 1733 at 600.degree. C. with compromises (see U.S. Pat. No. 5,110,748 issued May 5, 1992 to K. Sarma). SOI transistors on glass substrates are particularly attractive for sensors and displays, although many other applications are possible such as actuators, high temperature electronics, optoelectronics, and radiation hard electronics.
A wide variety of techniques have been proposed for realizing thin silicon films compatible with high-performance devices on an insulating substrate. Due to the high temperature processing requirements of silicon (greater than 800.degree. C.), silicon-on-glass substrate processing has not been possible except on the so-called "high-temperature" glass, such as Corning 1729 glass, capable of withstanding greater than 800.degree. C. temperatures. Other glasses used in commercial applications, such as lap-top displays, cannot withstand temperature exposures greater than 600.degree. C., such as the Corning 7059 or other "low-temperature" glasses. Due to the high temperatures of silicon processing conventional silicon-on-glass techniques have relied on amorphous (a-Si) and polycrystalline (p-Si) materials which can be doped and treated at temperatures that the glass can withstand, but whose performance is decidedly inferior to single-crystal films. These prior approaches to forming silicon-on-insulator substrates are exemplified by U.S. Pat. No. 5,013,681 issued May 7, 1991 to D. J. Godbey et al. and the following articles: "Nanosecond Thermal Processing For Ultra-High-Speed Device Technology", T. W. Sigmon et al., Materials Research Society Symp. Proc., Vol. 158, 1990, pp. 241-153; "Low-Temperature Fabrication of p+-n Diodes with 300-.ANG. Junction Depth", K. H. Weiner et al., IEEE Electron Device Letters, Vol. 13, No. 7, July 1992, pp. 369-371; and "Monte Carlo Simulation of a 30 nm Dual-Gate MOSFET. How Short Can Si Go?" D. J. Frank et al., IEDM Technical Digest, December 1992, pp. 593-597.
A technique for forming single-crystal silicon on insulator and single-crystal devices is presented in U.S. Pat. No. 5,110,748 issued May 5, 1992 to K. Sarma. This approach suffers from the drawback that the sheet resistance of the implanted layers is high since these layers are annealed at 600.degree. C. The mobility of the ELO (epitaxial layer overgrowth) layer is also less than ideal due to the formation of grain boundaries over the dielectric of interest. Furthermore, the back interface of the silicon underneath the active device area has a cusp which may cause back gate control problems.
Recently, a silicon-on-glass process has been developed using pulsed laser doped silicon layers, as described and claimed in copending application Ser. No. 08/137,401, filed Oct. 18, 1993, entitled "A Method For Forming Silicon On A Glass Substrate". Also, a process has been developed for forming buried components in the silicon-on-glass substrate and for providing electrical contacts for the buried components using pulsed laser energy, as described and claimed in copending application Ser. No. 08/137,412, filed Oct. 18, 1993, entitled "Silicon On Insulator With Active Buried Regions". In addition, a process has been recently developed to provide crystalline silicon devices on glass substrates, as described and claimed in copending application Ser. No. 08/137,411, filed Oct. 18, 1993, entitled "Crystalline Silicon Devices on Glass".
While these recent efforts have resulted in a significant advance in the SOI technologies, there are significant advantages to utilizing the conventional silicon high temperature processing, but there is a need for the capability to produce microelectronic devices on glasses incapable of withstanding temperatures greater than 600.degree. C. These advantages relate to the immediate ability of silicon microelectronics firms to take advantage of this technology without significant capital investment. The present invention satisfies this need by providing a process in which a low-temperature glass substrate may be used in a silicon-on-insulator device. Basically, this is accomplished by first forming the microelectronic device components on a silicon substrate and then transferring them to a glass substrate. Thus, single-crystal silicon films can be utilized, instead of the previously used amorphous and polycrystalline silicon films, in SOI devices on glass.
The use of anodic bonding for sealing the silicon to the glass may cause electrical damage to the components fabricated by this process. It has been discovered that bypassing current through the areas of the silicon-on-glass wafers in which the transistors will not be formed, will eliminate possible damage to the components formed. This is accomplished by the deposition of a metal layer between the formed components and the silicon layer to be bonded to the glass substrate. It is thus seen that the present invention overcomes the problems of the above-referenced approaches to SOI technology, avoids the incompatibility of the silicon and glass processing temperatures, solves the potential problem of causing damage to the electrical components on the silicon, and thus advances the state of this technology.